1. Field of the Invention
The present invention relates to a driving apparatus of a vibration type actuator such as a vibration type motor for applying a driving force using a vibration energy generated on a vibration member by applying an alternating signal to an electro-mechanical energy conversion element, and a device and image forming apparatus using the vibration type actuator and, more particularly, to an apparatus using the vibration type actuator for driving a member which requires high rotational speed precision in an electrophotographic image forming apparatus, such as a photosensitive drum serving as an image carrier in a copying machine.
2. Related Background Art
Conventionally, the speed of a vibration type actuator, particularly, a vibration type motor is controlled by proportional plus integral control or integral control, like other motors.
For example, in Japanese Patent Application Laid-Open No. 7-131987, as shown in FIG. 5, AC voltages, as alternating signals whose phases are shifted by 90.degree. by a 90 degree phase shifter 40, are applied from input power amplifiers 39 and 41 via matching coils 42 and 45 and driving electrodes 43 and 46 to a piezoelectric element 56 constituting a vibration member having two phases of a driving portion. One and the other driving signals are also used to detect a vibration state. The sum of the two signals is input to one input terminal of a differential amplifier 50 via capacitances 44 and 47, while a signal (ground voltage) from the output terminal of the piezoelectric element 56 is input to the other input terminal of the differential amplifier 50. The differential amplifier 50 outputs rotational speed information. The output from the differential amplifier 50 is input to one input terminal of another differential amplifier 52 via a rectifier 51, whereas a target speed value from a target speed setting means 53 is input to the other input terminal of the differential amplifier 52. The differential amplifier 52 inputs a speed difference to an integrator 55.
An adder 54 adds the difference value from the differential amplifier 52 and the integral value from the integrator 55 to adjust the driving frequency of the vibration type motor output from a voltage control oscillator 38 using the sum, thereby controlling the speed.
When a vibration type motor is used to drive the photosensitive drum of a copying machine, the printing precision is influenced by errors in moving distance, i.e., rotational angle of the surface of the photosensitive drum. In this prior art, if the speed varies due to disturbance or the like, the steady-state error (.DELTA.v) of the speed can be finally eliminated as for the speed deviation, as shown in FIG. 4A, but the steady-state error (.DELTA.x) of the dimension at a position obtained by integral of the speed cannot be eliminated, as shown in FIG. 4B. That is, a position shift caused by variations in speed cannot be corrected (upon variations in speed during movement between two points,. a target position cannot be attained within a predetermined time even if the speed returns to a target speed). As a result, high-quality printing cannot be performed.
More specifically, in single-color copying, a toner image on the photosensitive drum to be transferred onto a transfer member at a transfer position cannot be transferred at a correct position on the transfer member.
In color copying, when photosensitive drums carrying toner images of respective colors (four colors) are arranged in parallel, toner images transferred at different positions on a transfer member cause color misregistration.
Further, in the above prior art, proportional plus integral control is performed in analog thereby failing to provide high-precision control.